An aircraft radar altimeter system used during flight receives return signals that are Doppler shifted due to the aircraft's velocity. For any given velocity, the further ahead the tracked terrain or object is from the aircraft, the larger the Doppler shift in the return signal. Maximum Doppler shift occurs when the aircraft is flying directly towards the detected terrain or object. Typical radar altimeter systems filter out any return signals outside of a maximum Doppler frequency range of interest. Bandwidths and filters in the altimeter system are set to respond to the maximum Doppler shift associated with what terrain would be visible up to the particular aircraft's cruising velocity.
The radar altimeter's filter input typically has white noise across the entire frequency range. Narrowing the frequency range of interest reduces noise in the altimeter system. Therefore, setting the radar altimeter system to filter out any return signals outside of a frequency range of interest improves the signal to noise ratio (SNR). However, when the frequency range is narrowed, the ability of the system to track return signals (corresponding to terrain features) suffers.
For the reasons stated above and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the specification, there is a need in the art for a radar altimeter system that can track a return signal over the entire range of possible Doppler shift frequencies at any velocity with an improved signal to noise ratio.